Zeolitic para-xylene separation with tetralin derivatives as heavy desorbent

ABSTRACT

A chromatographic process able to separate paraxylene from C 8  isomers and C 9  aromatics. In the process, the para-xylene-containing feed mixture is contacted with an X or Y zeolite adsorbent having Group IA or IIA cations, e.g., barium and/or potassium, at exchangeable cationic sites while maintaining the water content of the adsorbent at about 4.2-6% (LOI). The para-xylene components are selectively adsorbed onto the adsorbent. The non-adsorbed feed is then removed from the adsorbent and the para-xylene recovered by desorption with tetralin or alkyl derivatives of tetralin. The C 9  &#39;s and the other xylene isomers in the raffinate, can be separated from this heavy desorbent by fractionation of the raffinate and the desorbent recycled to the process.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S.S.N. 422,369, filed Oct. 16, 1989, now U.S. Pat. No. 5,057,643, which is a continuation-in-part of U.S.S.N. 197,787 filed May 22, 1988, now U.S. Pat. No. 4,886,930 issued Dec. 12, 1989.

FIELD OF THE INVENTION

The field of art to which the claimed invention pertains is hydrocarbon separation. More specifically, the invention relates to a process for separating paraxylene from a feed mixture comprising at least two xylene isomers, including the para-isomer, which process employs a zeolitic adsorbent and particular desorbents. It is particularly advantageous in a process in which the feed contains C₉ aromatic hydrocarbon.

BACKGROUND OF THE INVENTION

In numerous processes described in the patent literature, for example U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 to deRosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback, 3,997,620 to Neuzil and 4,014,949 to Hedge, particular zeolitic adsorbents are used to separate the para isomer of dialkyl substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. Many of the above patents use benzene, toluene, or p-diethylbenzene as the desorbent. P-diethylbenzene (p-DEB) has become a commercial standard for this separation. However, p-DEB is a "heavy" desorbent (higher boiling than p-xylene) which suffers in the process for separating p-xylene from feed mixtures containing C₉ aromatics because the boiling point of p-DEB is too close to the boiling point of C₉ aromatics in the feed. Because the C₉ aromatics are difficult to separate from p-DEB by simple fractionation, the C₉ aromatics would gradually build up in the desorbent, which must be recycled for economic reasons. In the commercial process for recovering p-xylene from feed mixtures containing isomers, therefore, it has been necessary to reduce C₉ aromatics in the feed to below about 0.1% prior to the adsorptive separation of p-xylenes. This is usually done by distillation in a so-called xylene splitter column. Of course, substantial costs associated with this practice, such as capital costs of the xylene splitter and utilities necessary to achieve substantially complete removal of the C₉ aromatics, could be reduced greatly or eliminated if it were not necessary to first remove C₉ aromatics. Thus, while U.S. Pat. No. 3,686,342, supra, mentions tetralin as a possible heavy desorbent for the para-xylene separation process, that reference clearly states that p-DEB is the best desorbent for the separation and, further, does not address the problem that the preferred desorbents may have in separating feeds containing C₉ aromatics. Therefore, a higher boiling point material, that meets the selectivity requirements for desorbents and can be separated from C₉ aromatics, has been long sought and is still desirable.

It is also known that crystalline aluminosilicates or zeolites are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.

The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.

The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Pat. No. 4,159,284 or in a cocurrent, pulsed continuous process, like that disclosed in Gerhold U.S. Pat. Nos. 4,402,832 and 4,478,721.

Also, in some cases illustrated herein, it is necessary to remove three product streams in order to obtain a desired product intermediate in adsorption strength from an extract and a raffinate stream. This intermediate stream can be termed a second raffinate stream, as in U.S. Pat. No. 4,313,015 or a second extract stream, as in U.S. Pat. No. 3,723,302, both incorporated herein by reference, the latter incorporating abandoned application Serial No. 100,105 filed December 21, 1970. This case pertains when a contaminating component in the feed, such as p-ethyltoluene, is more strongly adsorbed than the desired product, p-xylene. It is not always necessary to remove p-ethyltoluene from p-xylene, e.g., where terephthalic acid is the final product of the oxidation of p-xylene, since oxidation of p-ethyltoluene results in the same product. However, if it is desired to keep the concentration of the contaminating component in the product as low as possible, a first extract is taken off, high in concentration of the desired component and lower in the contaminating product followed by a second extract, withdrawn at a point in zone 3 between the desorbent inlet and the first extract point, containing a high concentration of the contaminant and a lower concentration of the desired product. It is not necessary, however, to use a second desorbent, if the desorbent is able to first desorb the lightly held product and then desorb the remaining more strongly held contaminants, as disclosed in the aforementioned abandoned application. If the contaminating component in high concentrations and purity is desired, this can be achieved by withdrawing a second extract in the cocurrent pulsed batch process mentioned above.

The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.

I have discovered a process for employing a zeolite adsorbent for the separation of p-xylene from its isomers and, particularly, desorbents which are a substantial improvement in a process for separating xylene isomers where the feed mixture also contains C₉ aromatic impurities. I have further discovered that the water content of the adsorbent has important effects on mass transfer rates and selectivities which may vary with the desorbent selected.

SUMMARY OF THE INVENTION

In brief summary, the invention is a chromatographic process for separating p-xylene from a feed mixture comprising p-xylene and C₉ aromatic hydrocarbons and optionally, one or more additional xylene isomers (including ethylbenzene) comprising contacting said feed mixture with an X- or Y-type zeolite exchanged with Group IA or IIA metal ions at exchangeable cationic sites to effect the selective adsorption of said p-xylene and produce a raffinate comprising the other xylene isomers, including ethylbenzene, and C₉ aromatics. P-xylene is recovered thereafter by contacting the resulting para-xylene rich adsorbent with a desorbent comprising an alkyl or dialkyl derivative of 1,2,3,4-tetrahydronaphthalene (tetralin) or mixtures thereof. The desorbents are higher boiling than the C₉ aromatics, making it possible to separate the C₉ aromatics from the desorbent by simple fractionation so that the desorbent can be reused in the process without building up C₉ aromatics in the recycled desorbent. The water content of the adsorbent is an important factor in optimizing the separation ability of the adsorbent with specific tetralin derivatives, particularly with respect to mass transfer rates and selectivities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chromatographic representation of the separation of p-xylene from a mixture of xylene isomers and C₉ aromatics with a K-exchanged Y zeolite and a desorbent comprising a 30/70 mixture of tetralin and n-heptane.

FIG. 2 is similar to FIG. 1 except that the adsorbent is BaX.

FIG. 3 is similar to FIG. 1 except that the desorbent diluent is n-octane and shows the chromatographic separation of para-ethyl toluene and para-xylene from other C₈ and C₉ aromatics.

FIG. 4 is similar to FIG. 2 except that the desorbent diluent is n-octane and shows the separation of p-xylene from p-ethyltoluene and other isomers of xylene.

FIG. 5 is a graph plotting mass transfer time constants against adsorbent water content (LOI) for the separation of para-xylene from a C_(8/) C₉ aromatic mixture with BaX adsorbent and various methyl tetralin isomers and para-diethylbenzene desorbents.

In the figures, numerals are used to indicate the component lines in the graph of relative concentrations vs. retention volume (ml.) as follows: the tracer 1; mesitylene.-2; m-xylene 3; n-propylbenzene 4; o-xylene 5; 1,2,3-trimethylbenzene 6; ethylbenzene 7; 1,2,4-trimethylbenzene 8; cumene 9; p-xylene 10 and p-ethyltoluene 11.

DETAILED DESCRIPTION OF THE INVENTION

Adsorbents to be used in the process of this invention comprise specific crystalline aluminosilicates or molecular sieves, namely X and Y zeolites. The zeolites have known cage structures in which the alumina and silica tetrahedra are intimately connected in an open three-dimensional network to form cage-like structures with window-like pores. The tetrahedra are cross-linked by the sharing of oxygen atoms with spaces between the tetrahedra occupied by water molecules prior to partial or total dehydration of this zeolite. The dehydration of the zeolite results in crystals interlaced with cells having molecular dimensions and thus, the crystalline aluminosilicates are often referred to as "molecular sieves" when the separation which they effect is dependent essentially upon differences between the sizes of the feed molecules as, for instance, when smaller normal paraffin molecules are separated from larger isoparaffin molecules by using a particular molecular sieve. In the process of this invention, however, the term "molecular sieves", although widely used, is not strictly suitable since the separation of specific aromatic isomers is apparently dependent on differences in electrochemical attraction of the different isomers and the adsorbent rather than on pure physical size differences in the isomer molecules.

In hydrated form, the crystalline aluminosilicates encompass type X zeolites which are represented by Formula 1 below in terms of moles of oxides:

FORMULA 1

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 (2.5±0.5)SiO.sub.2: yH.sub.2 O

where "M" is a cation having a valence of not more than 3 which balances the electrovalence of the tetrahedra and is generally referred to as an exchangeable cationic site, "n" represents the valence of the cation, and "y", which represents the moles of water, is a value up to about 9 depending upon the identity of "M" and the degree of hydration of the crystal. As noted from Formula 1, the SiO_(2/) Al₂ O₃ mole ratio is 2.5±0.5. The cation "M" may be monovalent, divalent or trivalent cations or mixtures thereof.

The type Y structured zeolite, in the hydrated or partially hydrated form, can be similarly represented in terms of moles of oxides as in Formula 2 below:

FORMULA 2

    (0.9±0.2)M.sub.2/ nO:Al.sub.2 O.sub.3: wSiO.sub.2: yH.sub.2 O

where "M", "n" and "y" are the same as above and "w" is a value greater than about 3 up to about 6. The SiO_(2/) Al₂ O₃ mole ratio for type Y structured zeolites can thus be from about 3 to about 6. For both zeolites, the cation "M" may be one or more of a variety of cations but, as the zeolites are initially prepared, the cation "M" is also usually predominately sodium. The type Y zeolite containing predominately sodium cations at the exchangeable cationic sites is, therefore, referred to as a sodium-exchanged type-Y, or NaY, zeolite. Depending upon the purity of the reactants used to make the zeolite, other cations mentioned above may be present, however, as impurities.

The zeolites useful in the invention are typical as described above. However, the exchange of the cation of the as-manufactured zeolite by ions from Group IA or IIA, e.g., barium or potassium or mixtures thereof, is necessary to achieve the separation.

Typically, adsorbents used in separative processes contain the crystalline material dispersed in an amorphous, inorganic matrix or binder, having channels and cavities therein which enable liquid access to the crystalline material. Silica, alumina, clay or mixtures thereof are typical of such inorganic matrix materials. The binder aids in forming or agglomerating the crystalline particles which otherwise would comprise a fine powder. The adsorbent may thus be in the form of particles such as extrudates, aggregates, tablets, macrospheres or granules having a desired particle range, preferably from about 16 to about 60 mesh (Standard U.S. Mesh) (1.9 mm to 250 microns):

Feed mixtures which can be utilized in the process of this invention will comprise para-xylene, at least one other C₈ aromatic isomer, and may also contain one or more C₉ aromatics as impurities. Mixtures containing substantial quantities of para-xylene and other C₈ aromatic isomers and C₉ aromatics generally are produced by reforming and isomerization processes, processes which are well known to the refining and petrochemical arts. Many of the C₉ aromatics have boiling points in the range of 160-170° C. and cannot be easily removed by distillation from the standard desorbent, p-diethylbenzene. In the current process, therefore, the C₉ 's are usually removed from the feed by distillation before the adsorptive separation and consequent contact with the normal desorbent. I have discovered a desorbent which can be easily separated from the C₉ aromatics by fractionation and does not require the large column and quantity of utilities to pretreat the feed, resulting in substantial cost savings.

Reforming processes can provide feed mixtures for the process of this invention. In reforming processes, a naphtha feed is contacted with a platinum-halogen-containing catalyst at severities selected to produce an effluent containing C₈ aromatic isomers. Generally, the reformate is then fractionated to concentrate the C₈ aromatic isomers in a C₈ fraction which will contain the C₈ aromatic isomers as well as C₈ nonaromatics and C₉ aromatics. Feed mixtures for the process of this invention may also be obtained from isomerization and transalkylation processes. Xylene mixtures which are deficient in one or more isomers can be isomerized, at isomerization conditions, to produce an effluent containing C₈ aromatic isomers, e.g., enriched in p-xylene, as well as C₈ nonaromatics and C₉ aromatics. The C₉ aromatic content of isomerized xylene isomers can be as much as 1-2 wt.% depending on isomerization conditions. Likewise, transalkylation of mixtures of C₇ and C₉ aromatics produces xylene isomers. In all of these catalytic routes, the xylene splitter column must be employed to remove C₉ aromatics from C₈ aromatics before conventional adsorptive xylene separation methods can be employed. Thus, the feed mixtures to the process of this invention can contain quantities of C₉ aromatics and may also contain quantities of straight or branched chain paraffins, cycloparaffins, or olefinic material. It is preferable to have these quantities at a minimum amount in order to prevent contamination of products from this process by materials which are not selectively adsorbed or separated by the adsorbent. Preferably, the above-mentioned contaminants should be less than about 20% of the volume of the feed mixture passed into the process.

To separate the para-xylene from a feed mixture containing para-xylene, at least one other C₈ aromatic and C₉ aromatics, the mixture is contacted with the adsorbent at adsorption conditions and the para-xylene (and paraethyltoluene, if present) is more selectively adsorbed and retained by the adsorbent while the other components are relatively unadsorbed and are removed from the interstitial void spaces between the particles of adsorbent and from the surface of the adsorbent. The adsorbent containing the more selectively adsorbed paraxylene is referred to as a "rich" adsorbent--rich in the more selectively adsorbed para-xylene. The para-xylene is then recovered from the rich adsorbent by contacting the rich adsorbent with a desorbent material at desorption conditions.

In this process, which employs zeolitic adsorbents and which is generally operated continuously at substantially constant pressures and temperatures to ensure liquid phase, the desorbent material relied upon must be judiciously selected to satisfy several criteria. First, the desorbent material should displace an extract component from the adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent the extract component from displacing the desorbent material in a following adsorption cycle. Secondly, the desorbent material must be compatible with the particular adsorbent and the particular feed mixture. More specifically, they must not reduce or destroy the critical selectivity of the adsorbent for an extract component with respect to the raffinate component or react chemically with the feed components. Desorbent materials should additionally be easily separable from the feed mixture that is passed into the process. Both the raffinate components and the extract components are typically removed from the adsorbent in admixture with desorbent material and without a method of separating at least a portion of desorbent material, the purity of the extract product and the raffinate product would not be very high nor would the desorbent material be available for reuse in the process. It is, therefore, contemplated that any desorbent material used in this process will have a substantially different average boiling point than that of the feed mixture or any of its components, i.e., more than about 5° C. difference, to allow separation of at least a portion of the desorbent material from feed components in the extract and raffinate streams by simple fractional distillation, thereby permitting reuse of desorbent material in the process.

Finally, desorbent materials should be readily available and reasonable in cost. However, a suitable desorbent or desorbents for a particular separation with specific adsorbent are not always predictable. In the preferred isothermal, isobaric, liquid-phase operation of the process of this invention, when the feed material to the separation process contains more than about 0.1 wt.% C₉ aromatics. I have found that a desorbent material comprising tetralin or alkyl or dialkyl derivatives of tetralin or mixtures- thereof will desorb the extract from the adsorbent and can be separated from C₉ 's by distillation.

Suitable alkyl-substituted derivatives of tetralin include methyl tetralin, ethyl tetralin, propyl tetralin, isopropyltetralin, etc. Suitable dialkylsubstituted derivatives of tetralin include methyl ethyl tetralin, dimethyl tetralin, diethyltetralin, etc. Mixtures of tetralin with one or more of these derivatives, as well as mixtures of these derivatives also may be used with good results. In addition to tetralin, methyl tetralin and 2,6-dimethyl tetralin are preferred as desorbents in this process. Most preferred as desorbents in the separation are 1-methyl tetralin and 6-methyl tetralin. All position isomers and mixtures are intended to be included when any tetralin derivative is referred to herein.

A practical separation with certain alkyl derivatives of tetralin is enhanced through the amount of water present in the adsorbent. It has been determined that mass transfer rates (primarily, the speed with which the adsorbate may be desorbed) can be optimized through the control of the water content of the adsorbent within the range of from 4.2 to 6%, preferably 4.4 to 5.5%, as determined by LOI method at 500° C. The mass transfer rate (the reciprocal of the mass transfer time constant, 1/Kk), which is an overall rate including both adsorption and desorption rates, can be calculated from a moments analysis by the method described in Principles of Adsorption and Adsorption Processes, Ruthven, John Wiley and Sons, 1984, at pages 242-4, using the data obtained from the pulse test to calculate the first and second moments, σ (mean retention time of peak) and σ² (σ=standard deviation of peak), respectively.

It was noted, in general, that selectivity for para-xylene with respect to ethylbenzene (the limiting case) decreases with increasing water content while the mass transfer rates increase with increasing water content. The selectivity (βp/e) with the methyl tetralins through the range of 1-6% LOI is consistently greater than that of the standard heavy desorbent, para-diethylbenzene (p-DEB). Thus, the improved separation with the desorbents of the invention can be enhanced further for commercial application by control of the water level on the adsorbent within the aforesaid ranges. For example, the preferred alkyl tetralin, 1-methyl-tetralin, has selectivity (βp/e) of about 1.6-1.8 at LOI's of 4.4 to 5.5 whereas βp/e with p-DEB ranges from about 1.4 to 1.6 at LOI's in the range between 3 and 6. The mass transfer rate with 1-methyl tetralin is equivalent to p-DEB in the preferred LOI range. It is further noted, for 1-methyl tetralin, that retention volumes in the preferred LOI range, do not vary greatly with changes in LOI, thus, evidencing a stable separation system, insensitive to minor variations of water content within the operating ranges. Another preferred desorbent, 6-methyl tetralin, has a high selectivity and mass transfer rate at LOI's of about 5 to 5.8.

In the continuous process disclosed herein, it may also be necessary to measure the water content from time to time and control the level by adding moisture to the system, as required to maintain the desired water level. Additional water could be, for example, added with the feed.

Adsorption conditions will include a temperature range of from about 20 to about 250° C. with about 60 to about 200° C. being more preferred and a pressure just sufficient to maintain liquid phase, which may be from about atmospheric to 600 psig. Desorption conditions will include the same range of temperatures and pressure as used for adsorption conditions used

A dynamic testing apparatus is employed to test various adsorbents and desorbent material with a particular feed mixture to measure the adsorbent characteristics of adsorptive capacity and exchange rate. The apparatus consists of a helical adsorbent chamber of approximately 70 cc volume having inlet and outlet portions at opposite ends of the chamber. The chamber is contained within a temperature control means and, in addition, pressure control equipment is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative equipment, such as refractometers, polarimeters, chromatographs, etc., can be attached to the outlet line of the chamber and used to analyze, "on-stream", the effluent stream leaving the adsorbent chamber.

A pulse test, performed using this apparatus and the following general procedure, is used to determine data, e.g., selectivities, for various adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent by passing the desorbent material through the adsorbent chamber. At a convenient time, a pulse of feed containing known concentrations of a tracer and of a particular extract component or of a raffinate component, or both, all diluted in desorbent material is injected for a duration of several minutes. Desorbent flow is resumed, and the tracer and the extract and raffinate components are eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed by on-stream chromatographic equipment and traces of the envelopes of corresponding component peaks developed. Alternatively, effluent samples can be collected periodically and later analyzed separately by gas chromatography.

From information derived from the test, adsorbent performance can be rated in terms of void volume, retention volume for an extract or a raffinate component, and the rate of desorption of an extract component from the adsorbent and selectivity. Void volume is the non-selective volume of the adsorbent, which is expressed by the amount of desorbent pumped during the interval from initial flow to the center of the peak envelope of the tracer. The net retention volume (net) of an extract or a raffinate component may be characterized by the distance between the center of the peak envelope (gross retention volume) of the extract or raffinate component and the center of the peak envelope (void volume) of the tracer component or some other known reference point. It is expressed in terms of the volume in cubic centimeters of desorbent material pumped during this time interval represented by the distance between the peak envelopes. The rate of exchange or desorption rate of an extract component with the desorbent material can generally be characterized by the width of the peak envelopes at half intensity. The narrower the peak width, the faster the desorption rate. The desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of an extract component which has just been desorbed. This distance is again the volume of desorbent material pumped during this time interval. Selectivity, β, is determined by the ratio of the net retention volumes of the more strongly adsorbed component to each of the other components.

The following non-limiting examples are presented to illustrate the process of the present invention and are not intended to unduly restrict the scope of the claims attached hereto.

EXAMPLE I

In this experiment, a pulse test, using the apparatus as described above, was performed to evaluate the ability of the present invention to separate paraxylene (b.p. 138° C.) from the other xylene isomers and ethylbenzene (b.p.'s. from 136-145° C.) and from C₉ aromatics. The adsorbent used was a Y faujasite exchanged with potassium, dried to yield a loss upon ignition (LOI) of 1.26% at 900° C. combined with 15 wt.% of an amorphous clay binder.

For each pulse test, the column was maintained at a temperature of 150° C. and at a pressure of 165 psig so as to maintain liquid-phase operations. Gas chromatographic analysis equipment was attached to the column effluent stream in order to determine the composition of the effluent material at given time intervals. The feed mixture employed for each test was 5 cc of a mixture containing 0.45 cc each of the xylene isomers and ethylbenzene and each of the following C₉ aromatics: cumene, n-propylbenzene, p-ethyltoluene, mesitylene, 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene. Normal nonane (0.45 cc) was used as a tracer and 4.95 cc desorbent material was added to the feed. The desorbent material comprised 30 vol.% tetralin with the remainder being n-C₇ paraffin. The operations taking place for the test were as follows: The desorbent material was run continuously at a rate of about 1.44 cc per minute. At some convenient time interval, the desorbent was stopped and the feed mixture was run for a 3.47 minute interval. The desorbent stream was then resumed and continued to pass into the adsorbent column until all of the feed aromatics had been eluted from the column as determined by chromatographic analysis of the effluent material leaving the adsorption column.

The results of the tests shown in Table 1 and the chromatographic tracing of FIG. 1 illustrate the invention. The table lists the gross retention volume (GRV) and net retention volume (NRV) for each component of the feed and the selectivity, B, for each component with respect to the reference, p-xylene.

                                      TABLE 1                                      __________________________________________________________________________                         Gross  Net                                                             FIG. 1  Retention                                                                             Retention                                                                             Selectivity                                                                             Boiling                             Component   Reference No.                                                                          Volume (ml)                                                                           Volume (ml)                                                                           β   Point (°C.)                  __________________________________________________________________________     n-Nonane    1       47.7   0.0    (Tracer)                                     Ethylbenzene                                                                               7       59.8   12.1   1.43     136                                 p-Xylene    10      65.0   17.3   1.00 (Ref.)                                                                             138                                 Cumene      9       63.3   15.7   1.11     153                                 o-Xylene    5       55.6   7.9    2.18     144                                 n-Propylbenzene                                                                            4       53.9   6.3    2.77     159                                 p-Ethyltoluene                                                                             11      75.2   27.5   0.63     162                                 Mesitylene  2       53.6   5.9    2.92     163                                 1,2,4-Trimethylbenzene                                                                     8       62.9   15.3   1.13     168                                 1,2,3-Trimethylbenzene                                                                     6       60.3   12.6   1.37     175                                 m-Xylene    3       53.0   5.3    3.26     139                                 __________________________________________________________________________

EXAMPLE II

Example I was repeated except that the zeolite was a barium-exchanged X-type zeolite dried to yield an LOI of 4.7% at 900° C.. The results are shown in FIG. 2 and the following Table 2.

                                      TABLE 2                                      __________________________________________________________________________                         Gross  Net                                                             FIG. 2  Retention                                                                             Retention                                                                             Selectivity                                  Component   Reference No.                                                                          Volume (ml)                                                                           Volume (ml)                                                                           β                                       __________________________________________________________________________     n-Nonane    1       42.1   0.0    (Tracer)                                     Ethylbenzene                                                                               7       53.5   11.3   1.86                                         p-Xylene    10      63.2   21.1   1.00 (Ref.)                                  Cumene      9       52.6   10.5   2.01                                         o-Xylene    5       50.1   8.0    2.64                                         n-Propylbenzene                                                                            4       50.9   8.8    2.40                                         p-Ethyltoluene                                                                             11      65.4   23.3   0.90                                         Mesitylene  2       45.4   3.2    6.50                                         1,2,4-Trimethylbenzene                                                                     8       53.4   11.2   1.88                                         1,2,3-Trimethylbenzene                                                                     6       50.9   8.8    2.40                                         m-Xylene    3       49.2   7.1    2.97                                         __________________________________________________________________________

EXAMPLE III

Another pulse test was run under the same conditions and with the same materials as Example I, except that the desorbent was diluted with n-octane instead of n-heptane and the tracer was n-C₆ instead of n-C₉. The feed was 2 cc of a solution containing 0.5 g each of the following materials: n-C_(6;) ethylbenzene, p-xylene, m-xylene, o-xylene and p-ethyltoluene. The adsorbent was dried to yield an LOI of 0.64%. The results are shown in FIG. 3 and the following Table 3.

                                      TABLE 3                                      __________________________________________________________________________                     Gross  Net    Peak Width                                               FIG. 3  Retention                                                                             Retention                                                                             at Half                                                                              Selectivity                                Component                                                                              Reference No.                                                                          Volume (ml)                                                                           Volume (ml)                                                                           Height (ml)                                                                          β                                     __________________________________________________________________________     n-C.sub.6                                                                              1       43.8   0      8.3   Tracer                                     Ethylbenzene                                                                           7       56.9   13.1   12.3  1.50                                       p-Xylene                                                                               10      63.5   19.7   11    (Ref.)                                     m-Xylene                                                                               3       49.1   5.3    11.2  3.72                                       o-Xylene                                                                               5       51.8   8      12.6  2.46                                       p-Ethyltoluene                                                                         11      78     34.2   16.9  0.58                                       __________________________________________________________________________

EXAMPLE IV

The pulse test of Example III was repeated except that the adsorbent was a barium-exchanged X-type zeolite dried to yield an LOI of 2% at 900° C. The results are shown in FIG. 4 and the following Table 4.

                                      TABLE 4                                      __________________________________________________________________________                     Gross  Net    Peak Width                                               FIG. 4  Retention                                                                             Retention                                                                             at Half                                                                              Selectivity                                Component                                                                              Reference No.                                                                          Volume (ml)                                                                           Volume (ml)                                                                           Height (ml)                                                                          β                                     __________________________________________________________________________     n-Hexane                                                                               1       39.4   0      9.9   Tracer                                     Ethylbenzene                                                                           7       49.1   9.7    18.5  2.84                                       p-Xylene                                                                               10      66.9   27.5   18.8  Refer.                                     m-Xylene                                                                               3       52.1   12.7   22.1  2.17                                       o-Xylene                                                                               5       49.4   10     19    2.75                                       p-Ethyltoluene                                                                         11      58.4   19     26.4  1.45                                       __________________________________________________________________________

EXAMPLE V

Example II was repeated except that the desorbent was a 30/70 wt.% mixture of methyl tetralin and heptane, the LOI of the adsorbent, BaX, was 4.02% at 500° C. and the column flow rate was 1.23 cc/min. The methyl tetralin was a 35/65 wt.% mixture of the two possible isomers, 2-methyl tetralin and 3-methyl tetralin. The results are shown in the following Table 5, indicating improved selectivity for p-xylene over ethylbenzene and slightly better selectivity over p-ethyl toluene.

                  TABLE 5                                                          ______________________________________                                                   Gross      Net                                                                 Retention  Retention  Selectivity                                    Component Volume (ml)                                                                               Volume (ml)                                                                               β                                         ______________________________________                                         n-C.sub.9 39.7       0.0        (Tracer)                                       Ethylbenzene                                                                             52.5       12.8       2.20                                           p-Xylene  67.9       28.2       1.00   (Ref.)                                  m-Xylene  47.3       7.6        3.71                                           o-Xylene  46.8       7.1        3.97                                           Cumene    51.1       11.3       2.50                                           n-Propyl- 47.7       7.9        3.57                                           benzene                                                                        p-Ethyltoluene                                                                           64.6       24.9       1.13                                           Mesitylene                                                                               43.2       3.5        8.06                                           1,2,4-Trimethyl-                                                                         48.9       9.1        3.10                                           benzene                                                                        1,2,3-Trimethyl-                                                                         46.1       6.3        4.48                                           benzene                                                                        ______________________________________                                    

EXAMPLE VI

A number of further pulse tests were made under conditions similar to Example V on the same feed (Table 5) with the same adsorbent, BaX at different levels of water on the adsorbent (all measured as LOI at 500° C.) and with several different individual isomers of methyl tetralin, i.e., 1-methyl tetralin (1-MTET), 2-methyl tetralin (2-MTET), 5-methyl tetralin (5-MTET), 6-methyl tetralin (6-MTET), and as a comparison, para-diethylbenzene (p-DEB). In each case, the desorbent was a 30/70 mixture of the tetralin derivative and n-heptane. The results are summarized in the following Table 6 and FIG. 5. The table sets forth the net retention volume (NRV) and width of the peak at one-half height (HW) for para-xylene (PX), the selectivity of para-xylene with respect to ethyl benzene (βp/e) and the mass transfer time constant (1Kk) at each LOI value for 1-, 2-, 5- and 6-methyl tetralin and p-DEB. The data points, 1/Kk vs. LOI for each of the tests are shown in FIG. 5.

                  TABLE 6                                                          ______________________________________                                         Para-xylene Separation with Alkyl Tetralin Desorbents                                  LOI @                            1/Kk,                                 Desorbent                                                                              500 C   PX NRV,cc  PX HW,cc                                                                               (βp/e)                                                                          sec                                   ______________________________________                                         1-MTET  2.47    1.7        7.5     1.44  55                                    1-MTET  3.08    21.1       26.9    3.39  140                                   1-MTET  5.16    25.4       10.2    1.76  15                                    1-MTET  5.43    26.6       9.7     1.64  13                                    1-MTET  4.44    26.8       10.4    1.81  13                                    2-MTET  1.73    29.8       24.0    2.93  66                                    2-MTET  2.60    38.8       18.8    2.35  24                                    2-MTET  5.16    35.7       13.7    1.81  14                                    2-MTET  5.12    34.2       12.9    1.67  14                                    2-MTET  4.44    38.0       14.5    1.88  16                                    5-MTET  2.47    2.2        8.7     1.40  64                                    5-MTET  3.08    22.2       20.4    2.25  70                                    5-MTET  5.16    28.9       11.1    1.61  14                                    5-MTET  5.43    28.6       11.8    1.52  16                                    5-MTET  4.44    28.2       12.1    1.65  16                                    5-MTET  4.55    28.6       11.9    1.61  16                                    6-MTET  1.30    3.4        10.2    1.67  76                                    6-MTET  3.19    16.0       18.1    2.36  76                                    6-MTET  5.16    26.9       10.2    1.81  13                                    6-MTET  5.76    29.2       13.7    1.87  20                                    6-MTET  4.44    24.4       13.9    2.16  24                                    P-DEB   3.79    29.9       11.0    1.50  13                                    ______________________________________                                    

From the above table and FIG. 5, it can be seen that the methyl tetralins have greater selectivity (βp/e) than p-DEB and equivalent mass transfer time constants within the preferred range of water content. The following Table 7 sets forth the results of one of the above pulse tests, in which the desorbent was a 30/70 mixture of 1-methyl tetralin and n-heptane and adsorbent LOI was 4.43%.

                  TABLE 7                                                          ______________________________________                                                       Gross      Net        Selec-                                                   Retention  Retention  tivity                                     Component     Volume (ml)                                                                               Volume (ml)                                                                               β                                     ______________________________________                                         n-C.sub.9     39.4       0.0        0.00                                       Ethylbenzene  54.2       14.8       1.81                                       p-Xylene      66.1       26.8       1.00                                       m-Xylene      48.9       9.5        2.82                                       o-Xylene      50.1       10.8       2.48                                       Cumene        55.4       16.1       1.66                                       n-Propylbenzene                                                                              49.9       10.5       2.55                                       p-Ethyltoluene                                                                               65.5       26.1       1.02                                       Mesitylene    48.4       9.0        2.98                                       1,2,4-Trimethylbenzene                                                                       56.3       16.9       1.59                                       1,2,3-Trimethylbenzene                                                                       53.6       14.2       1.89                                       ______________________________________                                     

What is claimed is:
 1. A process for separating p-xylene from a mixture comprising p-xylene, at least about 0.1% Cg aromatic hydrocarbons and at least one other isomer of xylene which process comprises contacting said mixture with an X or Y crystalline aluminosilicate zeolite adsorbent containing a Group IA or IIA metal ion at exchangeable cationic sites having a water content of 4.2 to 6%, as measured by loss on ignition (LOI), at adsorption conditions, to effect the selective adsorption of said p-xylene by said adsorbent and to produce a raffinate stream comprising the less strongly adsorbed C₉ hydrocarbons and said other xylene isomers and contacting said adsorbent with a desorbent comprising an alkyl- or dialkyl-substituted 1,2,3,4-tetrahydronaphthalene (tetralin) at desorption conditions to effect the removal of p-xylene from said adsorbent as an extract stream.
 2. The process of claim 1 wherein said adsorbent is an X zeolite exchanged with barium.
 3. The process of claim 1 wherein said adsorbent is an Y zeolite exchanged with potassium.
 4. The process of claim 1 wherein said desorbent is 1methyl tetralin and the water content of said adsorbent is 4.4-5.5%.
 5. The process of claim 1 wherein said desorbent is 6-methyl tetralin and the water content of said adsorbent is about 5 to 5.8%.
 6. A process for separating p-xylene from a feed mixture comprising p-xylene and at least about 0.1% Cg aromatic hydrocarbons which process comprises contacting said mixture with an adsorbent comprising an X or Y crystalline aluminosilicate zeolite containing a Group IA or IIA metal ions at exchangeable cationic sites having a water content of 4.2 to 6%, as measured by loss on ignition, at adsorption conditions, to effect the selective adsorption of said p-xylene by said adsorbent and to produce a raffinate stream comprising the less strongly adsorbed C₉ hydrocarbons and thereafter contacting said adsorbent with a desorbent comprising an alkyl- or dialkyl-substituted 1,2,3,4-tetrahydronaphthalene (tetralin) at desorption conditions to effect the removal of p-xylene from said adsorbent as an extract stream.
 7. The process of claim 6 wherein said exchange ions are selected from the group consisting of barium and potassium.
 8. The process of claim 6 wherein said desorbent is 1-methyl tetralin and the water content of said adsorbent is 4.4 to 5.5.
 9. The process of claim 6 wherein said desorbent is 6-methyl tetralin and the water content of said adsorbent is 5 to 5.8.
 10. In a process for separating para-xylene from a feed mixture comprising para-xylene and at least one other xylene isomer, which process comprises contacting, at adsorption conditions, said feed with an adsorbent comprising X or Y crystalline aluminosilicate containing a Group IA or IIA metal at exchangeable cationic sites which selectively adsorbs said para-xylene, removing said feed from said adsorbent, and recovering said para-xylene by desorption at desorption conditions with a desorbent material, the improvement comprising utilizing an alkyl- or dialkyl-substituted 1,2,3,4-tetrahydronaphthalene (tetralin) as desorbent to separate a feed additionally containing at least about 0.1% to one C₉ aromatic hydrocarbon isomers, recovering the less strongly adsorbed C₉ aromatic feed materials and said other xylene isomer in the raffinate and fractionating said raffinate to recover C₉ aromatic hydrocarbons, said desorbent material and said other xylene isomers and recycling said desorbent material to the desorption step of said process, the improvement comprising maintaining the water content of said adsorbent at 4.2 to 6%, as measured by loss on ignition (LOI). 